When WalSndCaughtUp, sleep only in WalSndWaitForWal().

When WalSndCaughtUp, sleep only in WalSndWaitForWal().

Before sleeping, WalSndWaitForWal() sends a keepalive if MyWalSnd->write
< sentPtr.  That is important in logical replication.  When the latest
physical LSN yields no logical replication messages (a common case),
that keepalive elicits a reply, and processing the reply updates
pg_stat_replication.replay_lsn.  WalSndLoop() lacks that; when
WalSndLoop() slept, replay_lsn advancement could stall until
wal_receiver_status_interval elapsed.  This sometimes stalled
src/test/subscription/t/001_rep_changes.pl for up to 10s.

Discussion: https://postgr.es/m/20200406063649.GA3738151@rfd.leadboat.com